1. Field of the Invention
This invention relates to projection screens and to a method for making projection screens.
2. Description of the Prior Art
Known projection screens can be subdivided into two groups:
Group (A) Diffuse scattering projection screens with a matte surface, whereby the brightness of the picture is substantially independent of the viewing angle.
Group (B) Selectively scattering projection screens which concentrate the light scattered on the screen surface within a particular solid angle, and specifically in the case of a first sub-group B.sub.1 in a solid angle which encloses the incident direction of the light, and in a second sub-group B.sub.2 in a solid angle enclosing the reflection direction of the light on the projection screen plane. Known projection screens with a bead-, lens- or prism-like structure belong to sub-group B.sub.1 and screens with a rough metallized surface belong to sub-group B.sub.2.
The light output and radiation characteristics of a projection screen can be represented in a polar diagram which shows the light intensity of the light scattered on the screen as a function of the viewing angle under particular lighting conditions. FIG. 1 of the accompanying drawings show such a polar diagram in which P is the projection screen plane, I the incident light direction and .phi. the viewing angle. The semi-circular curve A shows the radiation characteristics of a diffuse scattering projection screen of Group A, and the pear-shaped curve B the radiation characteristics of a known selectively scattering, high efficiency projection screen of Group B.
The polar diagram of FIG. 1 shows the radiation characteristics in only one plane. In order to obtain a complete picture regarding the radiation characteristics of a projection screen, the polar diagram must also be shown for other planes located in the incident light direction I. Isotropic projection screens show the same polar behaviour for all planes enclosing the incident light direction I in the case of vertical incidence of light, while anisotropic projection screens show a different polar behaviour for different planes.
In the case of an isotropic projection screen, a maximum light output is obtained if all the incident light is uniformally scattered back with a cone. In FIG. 2 of the accompanying drawings, the circular sector-shaped line C represents the radiation characteristic of such an ideal projection screen which uniformally scatters back the incident light within a scattering angle .beta..
A small scattering angle .beta. gives a bright projection image and correspondingly limits the area within which the image can be observed. A radiation characteristic in accordance with line C has not hitherto been achievable.
In many cases the viewers looking at a projection screen adopt a horizontally widely fanned-out seating arrangement limited vertically by a narrow band. The light scattered back above and below this narrow band is wasted. In such cases higher efficiency is obtained with an anisotropic projection screen which scatters back the incident light horizontally in a scattering angle .beta..sub.H and vertically in a narrower scattering angle .beta..sub.V than with an isotropic projection screen.